The present invention is directed to optically coupled sensors for measuring a parameter of a combustible liquid, in general, and more particularly, to an optically coupled sensor using error self-correcting measurement techniques.
In general, conventional parameter measurement sensors for use in combustible liquids have been made intrinsically safe by transmitting power and control signals to and receiving measurement representative signals from the sensors over some non-conductive communication path, like an optical fiber path, for example. In the U.S. Pat. No. 4,963,729, issued Oct. 16, 1990 and entitled xe2x80x9cOptically Powered Sensor System With Improved Signal Conditioningxe2x80x9d which is assigned to the same assignee as the instant application, a conventional capacitive probe for measuring fuel level in an aircraft fuel tank is sensed using electronics at or near the probe which are optically coupled to a remote controller. The controller includes an optical source which provides optical energy to the probe electronics over an optical fiber path. The probe electronics converts the optical energy into electrical energy which is stored for powering the probe electronics. When optical power is interrupted, the probe electronics performs two measurements of the capacitance value of the probe using an integrator and two comparators, one with a reference capacitor and one without. Two spaced apart pulses are generated from the comparators with each measurement. Each set of pulses are converted to optical energy using a light emitting diode (LED), for example, which is transmitted back to the remote controller over the optical fiber path during the period of optical power interruption. Optical power is then resumed until the next measurement sample. The remote controller computes a compensated measurement of liquid level from the timing of the two sets of pulses received during the sampling period.
The present invention provides a precision measurement of probe capacitance with a minimum of power, complexity and cost for the probe electronics. It also lowers power of the probe electronics well within the twenty (20) microjoule safety limit for fuel tank use proposed by some airlines, and uses integration techniques to eliminate all offset error caused by the probe circuits. Accordingly, the optically coupled sensor of the present invention is much more accurate and stable and lower in cost than such sensors currently being used. Conventional probe placement and compensation for fuel measurement in a tank need not be altered for an embodiment of the present invention.
In accordance with one aspect of the present invention, an optically coupled circuit is electrically coupleable to a capacitive probe transducer disposed in a liquid in a tank for sensing the capacitance of the probe transducer which is a measure of a parameter of the liquid. The optically coupled circuit comprises: a first converter circuit for receiving optical energy over a non-conductive path and for converting the optical energy into electrical energy at a predetermined voltage potential; means for developing first and second reference voltage potentials from the predetermined voltage potential; a dual slope integrator circuit coupleable to the probe capacitor for charging the probe capacitor during a first integration period and discharging the probe capacitor during a second integration period utilizing the first and second reference voltage potentials, the integrator circuit including a circuit for comparing capacitive voltage generated during the first and second integration periods with the first and second reference voltage potentials to generate timing signals for each integration period; and means for determining the capacitance of the probe transducer as a function of timing signals from two successive integration periods.
In accordance with another aspect of the present invention, an optically coupled sensor system for measuring a parameter of a liquid in a tank comprises: a capacitive probe transducer disposable in the liquid, the probe capacitance being a measure of the liquid parameter; and an optically powered circuit electrically coupleable to the capacitive probe transducer for sensing the capacitance thereof The optically powered circuit comprises: a first converter circuit for receiving optical energy over a non-conductive path and for converting the optical energy into electrical energy at a predetermined voltage potential, the optically powered circuit being powered by the electrical energy; means for developing first and second reference voltage potentials from the predetermined voltage potential; a dual slope integrator circuit coupleable to the probe capacitor for charging the probe capacitor during a first integration period and discharging the probe capacitor during a second integration period utilizing the first and second reference voltage potentials, the integrator circuit including a circuit for comparing capacitive voltage generated during the first and second integration periods with the first and second reference voltage potentials to generate timing signals for each integration period; and a second converter circuit for converting the timing signals into optical signals for transmission over the non-conductive path. The system includes means coupleable to the non-conductive path for receiving the optical timing signals and determining the capacitance of the probe transducer as a function of timing signals from two successive integration periods.
In accordance with yet another aspect of the present invention, a method of determining the capacitance of a capacitive probe disposed in a liquid within a tank, the capacitance being used for measuring a parameter of said liquid, the method comprising the steps of: receiving optical energy from a non-conductive path; converting the optical energy to electrical energy at a predetermined voltage potential; developing first and second reference voltage potentials from the predetermined voltage potential; charging and discharging the probe capacitor during respective first and second integration periods utilizing the first and second reference voltage potentials; generating timing signals for each of the first and second integration periods utilizing the first and second reference voltage potentials; and determining probe capacitance using timing signals of two successive integration periods.